Image sensors receive light into an array of photosensitive pixels. Each pixel may be formed of a number of cooperating elements including, for example, a lens, often called a “microlens,” a color filter which can block incoming light of a particular color(s) from reaching the photosensitive portion, and the photosensitive portion itself. These elements are typically formed on different physical levels of a substrate. Traditionally, the elements of the pixels should have their centers substantially exactly aligned. That is, the microlens, the color filter, and the photosensitive portion should each be substantially coaxial.
One of the difficulties in designing and implementing imaging systems is the optimization of individual pixels within a pixel array. The problem becomes significant in imaging applications receiving non-telecentric light, where different pixels of the array are illuminated with light having different chief ray angles, depending on the location of the pixel in the array. Examples of non-telecentric light sources include cameras for cellular phones and imaging handsets.
In non-telecentric applications, a pixel at the center of the array receives light with a chief ray angle of 0 degrees, while pixels at the corners of the array may receive light with chief ray angles up to about 30 degrees. In addition, the relative illumination of the imaging lens results in large (up to 50%) degradation of light intensity across the array. This means that signals output from pixels at an edge of the array can be significantly degraded.
The standard way to optimize pixel characteristics for non-telecentric light is to shift the microlens-color filter array in an effort to minimize signal degradation and color distortion. One method for calculating a lens shift, for minimizing the signal degradation caused by cross-talk among pixels, is disclosed in U.S. Pub. No. 2005/0061951, assigned to Micron Technology Inc., and incorporated herein by reference. The remaining signal degradation, such as the signal degradation due to the relative illumination of the imaging lens, can be compensated for by using digital lens shading correction techniques.
FIG. 1 illustrates the general idea of using a microlens-color filter array shift to optimize pixel performance for different chief ray angles of light from an imaging lens. As shown in FIG. 1, an image sensor 10 includes a microlens array 12, a color filter array 13, and a pixel array 14. Incoming light 11 is produced from an imaging lens 15, such that individual rays of light 11a, 11b, 11c, 11d strike the pixel array 14 at different angles. Rather than having a center of each microlens 12a aligned with a center of a respective color filter 13a and a corresponding pixel center 14a, the microlens array 12 and color filter array 13 are shifted with respect to each other, to focus the incoming light 11 onto underlying, photosensitive regions of the pixel array 14.
One way of calculating the microlens-color filter array shift and building a digital lens shading correction algorithm for the imager is based on the orthogonal X and Y coordinate system. The positions of the microlens-color filter array along the X and Y-axes are calculated using an imaging lens-chief ray angle function. Then, as illustrated in FIG. 2, the position of the microlens-chief ray angle for all other pixels within the array are calculated as a product of a corresponding shift along the X and Y coordinate axis. For example, the microlens shift (ΔS) for pixels [k,0], [0,1], [m,0], [0,n] are determined from the chief ray angle as a function of X and Y coordinates. The microlens shift for pixels [k,l] and [m,n] are determined using the following equations:ΔSk,1=square root((ΔSk,0)2+(ΔS0,1)2); where ΔSk,1 is the calculated shift for a pixel in column “k” and row “l,” andΔSm,n=square root((ΔSm,o)2+(ΔS0,n)2) where ΔSm,n is the calculated shift for a pixel in column “m” and row “n.”
The same method is used for calculating the correction coefficients in a digital lens shading correction algorithm. Signal processing circuitry applies the correction coefficient to the digitized versions of output signals that are received from pixels in the pixel array. This correction is done to account for the difference in illumination of a signal across the array.
This orthogonal method works well with imaging lenses that have linear dependence of the chief ray angle as a function of the relative image height (i.e., “field”). Imaging lenses that have non-linear behavior of the chief ray angle as a function of image height, however, have difficulty in using the orthogonal method. Specifically, the microlens-color filter locations and the lens shading correction can not be fully optimized for the imaging lens, which results in non-uniform signal response as well as color distortion.
FIGS. 3a-3b show graphs illustrating chief ray angle versus image height for two typical lens designs. FIG. 3a represents a lens with a linear chief ray angle as a function of field. FIG. 3b represents a lens with a non-linear chief ray angle as a function of field. The orthogonal optimization method described above results in good agreement between the desired and actual lens shift for the linear case, as shown in FIG. 4. FIG. 4 is a graph depicting the microlens shift needed and the actual microlens shift obtained using the orthogonal algorithms discussed above; as can be seen, the actual and needed shifts correspond well. On the other hand, there is a significant discrepancy in the needed and actual microlens shift for the non-linear case (FIG. 3b) of the imaging lens, as shown in FIG. 5. In this case, as the image height is increased, the discrepancy becomes greater between the actual and needed lens shift. This same discrepancy also occurs if the orthogonal method is used for the digital lens shading correction function as well.
Obtaining the optimized microlens and color filter array location for a pixel array is becoming increasingly important as modern technologies require a reduced pixel size with increased image quality. In addition, many imaging lenses for mobile applications, such as e.g., cellular telephones, which are becoming increasingly popular, have significant non-linear dependence of the chief ray angle as a function of field. The relative illumination curve as a function of field is non-linear for most of these applications as well. As stated above, traditional methods for optimizing these arrays are not sufficient.
Accordingly, there is a need and desire for a method of calculating microlens-color filter array shift for lenses having a non-linear correlation of chief ray angle and image height. An improved method of performing digital lens shading correction is also desired.